How to Mix Oil and Water

Scientists in Belgium have uncovered a new way to shake things up. Violent bouncing of a water droplet coated with oil causes the oil layer to move inside and fracture into many oily globs. In a paper published in the December Chaos, researchers at the University of Liège in Belgium call this microemulsion of oil and water the mayonnaise droplet.

From earlier experiments, researchers knew that oily droplets bounce several times on a bed of oil before merging with the oil base. But Denis Terwagne and colleagues wanted to know what would happen to an oil-coated water droplet if the bouncing was prolonged. To find out, the team constructed a moving base that would quickly raise and lower the oil bed, similar to using a horizontal Ping-Pong paddle to keep a ball bouncing.

When the oil-coated water droplet hit the oil surface and deformed, some of the outer oil layer was ushered into the interior of the droplet. Subsequent bounces shattered this interior oil glob, creating a thoroughly mixed oil-in-water-in oil droplet, the researchers found.

Understanding the forces that govern the mayonnaise droplet may help scientists design new microfluidic systems. Bouncing droplets could enable more efficient ways to create complex mixtures of liquids, such as for cosmetics or pharmaceuticals.